Forwarding Signal Supply Voltage in Data Transmission System

ABSTRACT

In a data transmission system, one or more signal supply voltages for generating the signaling voltage of a signal to be transmitted are generated in a first circuit and forwarded from the first circuit to a second circuit. The second circuit may use the forwarded signal supply voltages to generate another signal to be transmitted back from the second circuit to the first circuit, thereby obviating the need to generate signal supply voltages separately in the second circuit. The first circuit may also adjust the signal supply voltages based on the signal transmitted back from the second circuit to the first circuit. The data transmission system may employ a single-ended signaling system in which the signaling voltage is referenced to a reference voltage that is a power supply voltage such as ground, shared by the first circuit and the second circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application under 35 U.S.C. §121 of,and claims priority under 35 U.S.C. §120 from, co-pending U.S. patentapplication Ser. No. 13/391,223, having an effective filing date under35 U.S.C. §363 of Mar. 30, 2010 and a 35 U.S.C. §371 completion date ofFeb. 17, 2012, which is a U.S. national phase application under 35U.S.C. §371 of international application no. PCT/US2010/029252, havingan international filing date of Mar. 30, 2010, which claims priorityunder 35 U.S.C. §119(e) from U.S. Provisional Patent Application No.61/238,511, filed on Aug. 31, 2009, all of which are incorporated byreference herein.

BACKGROUND

The present disclosure relates to the generation and control of signalsupply voltages for use in a data transmission system.

In a communication system, data or other information may be transmittedfrom one integrated circuit (IC) device to another IC device as electricsignals over one or more wires. For example, a memory controller maytransmit (during a “memory write” operation) data as data signals to amemory device over one or more data communication channels, while thememory device may transmit (during a “memory read” operation) data asdata signals to the memory controller using the same or different datacommunication channels. In many instances, a signal from a transmittingdevice would have one or more characteristics (e.g., a signal swing)that are dependent on one or more regulated signal supply voltages.Conventionally, each IC device in a communication system obtains a powersupply voltage from an external voltage regulation device. That powersupply voltage is often used directly, but in some cases is regulatedagain, internal to the chip, to generate a signal supply voltage thatdetermines the one or more characteristics of the signals sent to otherdevices. In the above example involving the memory controller and thememory device, the memory controller would include a set of internalvoltage regulation circuitry to generate the regulated voltage(s) forforming the write data signals while the memory device would includeanother set of internal voltage regulation circuitry to generate theregulated voltage(s) for forming the read data signals.

Such arrangement would have several drawbacks. First, on-chip, internalvoltage regulators or generators require relatively complex circuitry(e.g., charge pumps) and/or involve relatively large circuit components(e.g., on-chip capacitors), and thus add to the size and manufacturingcost of the IC device. Second, when the communicating IC devices haveindependent voltage regulators to generate their own signal supplyvoltage(s), the IC devices that transmit the signal would not haveinformation to adjust one or more signal characteristics of the signalto suit the IC device receiving the signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the embodiments can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings.

FIG. 1A illustrates a data transmission system, according to oneembodiment.

FIG. 1B illustrates a data transmission system employing power supplyvoltage referenced single-ended signaling with forwarding of the signalsupply voltages, according to another embodiment.

FIG. 1C illustrates a data transmission system employing power supplyvoltage referenced single-ended signaling with forwarding and feedbackregulation of the signal supply voltages, according to still anotherembodiment.

FIGS. 2A, 2B, 2C, 2D, 2E, and 2F illustrate an example of a voltageregulator employed by the data transmission system of FIGS. 1A through1C, according to one embodiment.

FIG. 3 illustrate a receiver amplifier suitable for use in the datatransmission system of FIGS. 1A through 1C, according to one embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

The features and advantages described in the specification are not allinclusive and, in particular, many additional features and advantageswill be apparent to one of ordinary skill in the art in view of thedrawings, specification, and claims. Moreover, it should be noted thatthe language used in the specification has been principally selected forreadability and instructional purposes, and may not have been selectedto delineate or circumscribe the inventive subject matter.

The Figures (FIG.) and the following description relate to embodimentsby way of illustration only. It should be noted that from the followingdiscussion, alternative embodiments of the structures and methodsdisclosed herein will be readily recognized as viable alternatives thatmay be employed without departing from the principles of the presentdisclosure.

Reference will now be made in detail to several embodiments, examples ofwhich are illustrated in the accompanying figures. It is noted thatwherever practicable similar or like reference numbers may be used inthe figures and may indicate similar or like functionality. The figuresdepict embodiments for purposes of illustration only. One skilled in theart will readily recognize from the following description thatalternative embodiments of the structures and methods illustrated hereinmay be employed without departing from the principles of the embodimentsdescribed herein.

Embodiments of the present disclosure include a data transmission systemin which one or more signal supply voltages that determine one or moresignal characteristics are generated in a first circuit and forwarded(i.e., electrically driven) from the first circuit to a second circuit.The first or second circuit uses the signal supply voltages to generatesignals to be transmitted to the other circuit, thereby obviating theneed to generate signal supply voltages separately in the secondcircuit. The first circuit may also adjust the signal supply voltagesbased on the one or more signal characteristics of a signal transmittedback from the second circuit to the first circuit. In some embodiments,the data transmission system may employ a single-ended signaling systemin which the signal supply voltages determine a signal swing of thetransmitted signal One or more of these signal supply voltages may bereferenced to a reference voltage, such as a power supply voltage, suchas a ground voltage (GND), or another common reference voltage shared bythe first circuit and the second circuit.

Turning to the figures, FIG. 1A illustrates a data transmission systemin which signal supply voltages are forwarded from one circuit toanother circuit, according to one embodiment. Data transmission system100 includes circuits 100 a and 100 b, communicating with each other viaa communication link 131 that includes one or more transmission lines.Circuit 100 a includes a transmitter 102, a receiver 144, voltageregulators 140, 142, an optional feedback analyzer circuit 147, and atleast one terminal (or pin) 122 connected to communication link 131.Circuit 100 b includes a receiver 104, a transmitter 141, and at leastone terminals (pin) 125 connected to communication link 131. Althoughcommunication link 131 is shown to include a single transmission line inFIG. 1A and transmission system 100 is shown to use a single-endedsignaling scheme, communication link 131 may include multipletransmission lines to support a different signaling scheme such as adifferential signaling scheme. Furthermore, the system shown in FIG. 1Apresumes a bi-directional sharing of a transmission line, where circuits100 a and 100 b take turns (in a time-multiplexed fashion) being thereceiver while the other acts as the transmitter. In other embodiments,non time-multiplexed unidirectional signaling using multiple signallines may be used. Furthermore, the circuits 100 a and 100 b in system100 may be electrically interconnected in a variety of ways, includingvia electrical traces on a circuit board (e.g., a PC motherboard or PCgraphics card) or via electrical traces within the same multi-die ormulti-chip package (e.g., a package-on-package (POP) assembly, or athru-silicon via (TSV) stacked-die integrated circuit). Alternativechip-to-chip or die-to-die electrical interconnection means are readilyrecognized by one skilled in the art as viable alternatives that may beemployed without departing from the principles of the presentdisclosure.

Voltage regulators 140, 142 in circuit 100 a generate signal supplyvoltages V1, V2, respectively, which are used by transmitter 102 incircuit 100 a to form a signal for transmitting to circuit 100 b. Inother embodiments, only one voltage regulator may be necessary togenerate a single supply voltage V1, as one of the power-supply rails(e.g., GND) could be used to provide the other signal supply voltage V2.In one embodiment, system 100 further includes a voltage forwardingmechanism to forward (or drive) the signal supply voltages generated incircuit 100 a to circuit 100 b. The voltage forwarding mechanism mayinclude electrical conductors 146 and 148 that are coupled to circuit100 a via terminals or pins 124 and 126, respectively, and to circuit100 b via terminals or pins 128 and 129, respectively. The forwardedvoltages V1 and V2 are used by transmitter 142 to form a signal fortransmitting back to circuit 100 a. The data transmission system 100allows the signal supply voltages V1, V2 to be generated on one circuit100 a and be forwarded to the other circuit 100 b, thereby obviating theneed to include separate voltage regulation circuitry for generating thesignal supply voltages V1, V2 in circuit 100 b. In one embodiment,circuit 100 a is realized in a semiconductor process more amenable tocomplex circuits (such as is used for CPU's and memory controllers),while circuit 100 b is realized in a semiconductor process optimized forvery low cost production (such as is used for DRAM or Flash memorydevices). Also, circuits 100 a, 100 b may be any other type of ICs thatthat communicate with each other using signals.

In one embodiment, when circuit 100 a is transmitting to circuit 100 b,the signal formed by transmitter 102 is transmitted via line 131, andreceived by receiver 104, which recovers the data (Rdata 143) from thesignal. Conversely, when circuit 100 b is transmitting to circuit 100 a,the signal generated by transmitter 142 is transmitted back to circuit100 a via line 131 (when circuit 100 a and 100 b comprise abi-directional signaling system) or a different transmission line orlines (when circuit 100 a and 100 b comprise a unidirectional signalingsystem), and received by receiver 144 of circuit 100 a, which recoversthe data (Rdata) 145 from the signal.

The optional feedback analyzer circuit 147 in receiver 144 is configuredto analyze the characteristics of the signal received at the receiver144, and to generate control signals 149 for adjusting the voltage levelof the signal supply voltages V1, V2 generated by voltage regulators140, 142 (in other embodiments in which one of the power supplies isused as one of the signal supply voltages, only V1 may requiregeneration and adjustment). For example, feedback analyzer 147 may beconfigured such that, at system start-up, control signal 149 sets thesignal supply voltages V1, V2 at levels that result in the signal swingbeing close to zero. Then, feedback analyzer 147 varies control signal149 so that V1 and V2 are adjusted accordingly to gradually increase thesignal swing, until the quality of the signal received has reached adesired level. Alternatively, during system start-up, the signal supplyvoltages V1, V2 may be set at a level so that the signal swing is largerthan desired. Then, V1 and V2 may be adjusted so that the signal swingis gradually decreased, until the feedback analyzer circuit 147determines that the quality of the signal received has become lower thandesired. V1 and V2 can then be adjusted via control signal 149 so thatthe signal swing is brought back up slightly just to bring the qualityof the signal received back to a desired level. Thus, feedback analyzercircuit 147 provides a feedback loop for generating the signal supplyvoltages V1, V2 using voltage regulators 140, 142, so that a desirablesignal level in the signals between circuits 100 a and 100 b can bemaintained. Using the feedback analyzer circuit 147, signals withsmaller signal swings can be transmitted between the circuits 100 a and100 b, resulting in increased power efficiency in system 100 andprotecting receivers 104 and 144 from being damaged by overly stronginput signals. Feedback analyzer circuit 147 may include, for example, abit error rate (BER) detector that measures the bit error rate in thereceived signal RData 145.

In addition to allowing feedback control, as described in the previousparagraph, the data transmission system 100 of FIG. 1A has otherbenefits. First, as mentioned before, circuit 100 b, which may be in avery cost-sensitive circuit such as a memory device (e.g., a DRAM orFlash memory circuit) than circuit 100 a, does not need separate voltageregulators for generating its signal supply voltages. Thus, circuit 100b can be implemented in smaller size with simpler circuitry, and becheaper and more power-efficient.

Second, since the signal supply voltages V1, V2 are forwarded viaconductive paths (signal supply voltage rails) 146, 148, the datatransmission system 100 may include external (off-chip) capacitors 165,167 on the signal supply voltage rails 146, 148 to reduce noise that maybe present in the signal supply voltages V1, V2. These externalcapacitors 165, 167 may have rather large capacitances, for example,about 50 pF-100 pF. Without the forwarding of the signal supply voltagesvia external (off-chip) connections, such large capacitors 165, 167 forreducing noise would have to be added on-chip in the circuits 100 a, 100b, thereby significantly adding to the size and cost of manufacturingthe circuits 100 a, 100 b. The external (off-chip) connection of thesignal supply voltages V1, V2 allow these on-chip capacitors to bereplaced by large off-chip capacitors 165, 167, thereby reducing thesize and manufacturing cost of the ICs 100 a, 100 b significantly.

Third, in case of a deep power-down state in which the data transmissionsystem 100 is powered off, circuit 100 a can simply shut itself downwithout having to control circuit 100 b to shut down, because circuit100 b would stop signaling when the forwarded signal supply voltages areturned off.

FIG. 1B illustrates an embodiment of a bi-directional signaling system100 using single ended signaling where a power supply rail (such as aground plane) 130 is used to provide a reference voltage. In thisembodiment, two signal supply rails are needed: V1=+Vs and V2=−Vs.Voltage source(s) 140 is shown as comprised of a voltage source 140 thatprovides +Vs and a voltage source 142 that provides −Vs. Transmitter 102may include signal-controlled switching elements 108, 110 each coupledbetween a common drive node and respective signal supply voltages, +Vsand −Vs. Transmitter 102 further includes a resistive element 114coupled between the common drive node and terminal 133 to act as aseries source termination (e.g., to match a characteristic impedance,Z0, of the signal line). Thus the transmitter's internal Z0 impedance114 and the impedance of the line 132, terminated at Z0 by terminationimpedance 116, form a voltage divider that splits the two supplyvoltages, so that the signaling voltage 120 of signal on line 132toggles between about +Vs/2 and −Vs/2, as shown in FIG. 1B. In someembodiments, the functions of switches 108, 110 and resistor 114 may becombined in a single device, for example a MOS field effect transistor(MOSFET) operating in the linear or “triode” region of operation; insuch embodiments, the linear relationship of the voltage between thedrain and source terminals of the MOSFET and the current flowing betweendrain and source causes the device to operate in a mode approximating aresistor.

Still referring to FIG. 1B, receiver 104 may include a differentialamplifier or other comparator circuit 112. Amplifier 112 amplifies thetime-varying difference between the ground rail and signal potentials toproduce the received data signal (RData) 143 that corresponds to theoriginally transmitted data 106. As shown, a termination element(depicted as resistive element 116) may be coupled between the inputnodes 136 and 138 to terminate the incoming signal line according to thecharacteristic impedance, Z0.

In the embodiment of FIG. 1B, the GND potential is coupled to terminal(or power supply rail) 134 and is used as the reference voltage for thesingle-ended signaling scheme, while both +Vs and −Vs are regulatedsignal supply voltages forwarded from circuit 100 a to circuit 100 b.However, in other embodiments, +Vs may be the only regulated signalsupply voltage forwarded from circuit 100 a to circuit 100 b, andanother reference voltage (e.g., half of +Vs) may be used for thesingle-ended signaling scheme.

The embodiment of system 100 shown in FIG. 1B where ground referencedsingle-ended signaling scheme is used has several benefits. First, theGND plane 130 used as the reference voltage in IC to IC signalingsystems is attached to the reference plane in IC packages and circuitboards, and is generally the lowest-impedance network in a memorysystem. Since the reference output terminal 134 of transmitter 102 andthe reference input terminal 138 of receiver 104 are both connected tothis low impedance reference node (GND plane) 130, it is relatively easyto guarantee agreement on the reference level between various componentsof the memory system, including transmitter 102 and receiver 104.Another advantage of the transmission system 150 is that the signalcurrent sourced from transmitter 102 in circuit 100 a flows oversignaling line 132 to receiver 104 in circuit 100 b and then returns totransmitter 102 through the GND plane 130 via a separate path that maybe made parallel to the signaling line 132. Thus, the return currentflows anti-parallel to the signal current, so the signal current andsignal-return connections at the chip level are essentiallydifferential. Thus, noise and cross-talk that would otherwise begenerated by return current that is not parallel to the signal currentcan be avoided.

Despite the many advantages of the embodiment of system 100 shown inFIG. 1B, system 100 may employ other signaling schemes such as ones inwhich only a single supply voltage regulator and associated forwardingconductive path is needed, while a power supply rail (e.g., GND) is usedto generate the second signal supply voltage.

Still referring to FIG. 1B, as explained above, circuit 100 a includesvoltage regulators 140, 142 for generating the signal supply voltages+Vs, −Vs. Circuit 100 b does not include any voltage regulation circuitsfor generating the signal supply voltages +Vs, −Vs. Rather, the signalsupply voltage +Vs is forwarded from circuit 100 a to circuit 100 b viaterminal 124, line (supply voltage rail) 146, and terminal 128, and thesignal supply voltage −Vs is forwarded from circuit 100 a to circuit 100b via terminal 126, line (supply voltage rail) 148, and terminal 129.External capacitors 165, 167 for reducing noise in the signal supplyvoltages +Vs, −Vs can be connected to lines 146, 148 and the GND plane130 external to the ICs 100 a, 100 b, thereby obviating the need forlarge on-chip capacitors in circuits 100 a, 100 b.

Circuit 100 b uses the signal supply voltages +Vs, −Vs forwarded fromcircuit 100 a to generate signaling voltages on its own transmitter 141.Specifically, transmitter 141 includes switches 162, 163 and resistor161, and is connected to the signal supply voltages +Vs, −Vs that areforwarded from circuit 100 a. Transmitter 141 generates a signal 121that toggles between −Vs/2 and +Vs/2 and referenced to GND responsive toinput data 115, similarly to transmitter 102, using the signal supplyvoltages +Vs, −Vs forwarded from circuit 100 a.

The signal 121 generated by transmitter 141 is transmitted back tocircuit 100 a via terminal 136, line 132, and terminal 133, referencedto the GND plane 130, and received by receiver amplifier 144 of circuit100 a. Receiver amplifier 144 is connected to lines 132, 130 viaterminals 133, 134. Receiver amplifier 144 detects the difference involtages between the signaling voltage 121 on line 132 and the GNDvoltage on line 130 across resistance 152 to recover RData 145corresponding to the input data 115.

Although the embodiment of data transmission system 100 of FIG. 1B isshown as including a pair of terminals 124, 126 and a pair of terminals128, 129 for forwarding of the signal supply voltages +Vs, −Vs for eachsignal transmission line 132, it is possible for a plurality of signaltransmission lines 132 to share the same forwarded signal supplyvoltages +Vs, −Vs. For example, it is possible for a parallel bus of 8signal transmission lines 132 corresponding to 8-bit (or 1-byte) of datato share one pair of signal supply voltages +Vs, −Vs that are forwardedfrom circuit 100 a to circuit 100 b. The plurality of signaltransmission lines 132 may also share the same ground plane forproviding the reference voltage. In other embodiments, as describedabove, only a single supply voltage +Vs may be necessary, if the a powersupply such as GND is used to generate the second signal supply voltage.

In other embodiments, the communication link 131 may include a pluralityof signal transmission lines, circuit 100 a may include a plurality oftransmitters coupled to respective ones of the plurality of signaltransmission lines, and circuit 100 b may include a plurality ofreceivers coupled to respective ones of the plurality of signaltransmission lines to receive signals transmitted by respective ones ofthe plurality of transmitters, and the voltage forwarding mechanism mayinclude at least one electrical conductor coupled between the circuits100 a, 100 b for each of the plurality of signal transmission lines. Instill other embodiments, the plurality of signal transmission linesorganized in groups, and the voltage forwarding mechanism may includesat least one electrical conductor coupled between circuits 100 a, 100 bfor each group of the plurality of signal transmission lines. In stillother embodiments, the multiple transmission lines may be organized inpairs, circuit 100 a may include a plurality of transmitters coupled tothe respective pairs of the signal transmission lines, and circuit 100 bmay includes a plurality of receivers coupled to the respective pairs ofthe signal transmission lines to receive signals transmitted byrespective ones of the plurality of transmitters, and the voltageforwarding mechanism may includes at least one electrical conductorcoupled between circuits 100 a, 100 b for each pair of the signaltransmission lines.

FIG. 1C illustrates another embodiment of the data transmission system100 employing power supply voltage referenced single-ended signalingwith signal supply voltage forwarding and feedback regulation, accordingto still another embodiment. The data transmission system 100 of FIG. 1Cis similar to the data transmission system 100 of FIG. 1B, except that afeedback loop comprised of a feedback analyzer circuit 147 is used incircuit 100 a.

More specifically, feedback analyzer 147 receives RData 145 receivedfrom circuit 100 b and determines a bit error rate of the receivedRData. A bit error rate (BER) refers to the number of erroneous bits inRData 145 during a time period divided by the total number of bits inRData 145 during the time period. Any conventional scheme of bit errorrate detection can be used in the feedback analyzer 147 itself to detectthe BER of the received RData 145 (e.g., circuit 100 b could beconfigured to transmit a known, auto-generated PRBS pattern, and circuit100 a would be configured to count the errors in that known pattern).Since the signal supply voltages +Vs, −Vs are relatively small voltages(e.g., 100 mV, −100 mV), the signal generated by transmitter 141 usingthe signal supply voltage +Vs, −Vs forwarded from circuit 100 a may becorrupted by normal system noise. The determined bit error rate isindication of such noise overwhelming the signal supply voltage +Vs,−Vs. When feedback analyzer 147 determines that the BER in the receiveddata Rdata 145 is higher than desired, feedback analyzer 147 mayincrease a reference voltage Vref provided to voltage regulator 140, 142such that the signal supply voltages +Vs, −Vs generated by voltageregulator 140, 142 are increased. The increased signal supply voltages+Vs, −Vs are forwarded from circuit 100 a to circuit 100 b, and in turnlower the BER of the signal 121 generated by transmitter 141 of circuit100 b (by providing more signal to counteract the system noise). On theother hand, feedback analyzer 147 may decrease the reference voltageVref provided to voltage regulators 140, 142 from a higher than desiredlevel such that the signal supply voltages +Vs, −Vs generated by voltageregulators 140, 142 and the voltage swing of the signal 121 generated bytransmitter 141 are decreased from a higher than desired level to aminimal level needed to maintain an acceptable BER. Thus, the feedbackanalyzer 147 completes a feedback loop for the generation of the signalsupply voltages by voltage regulators 140, 142 in order to maintain adesirable BER of the received RData 145 without using too large a signalswing.

In one embodiment, voltage regulators 140, 142 are charge pumps. FIG. 2Aillustrates one embodiment of a voltage source 200 that includes thecharge pumps 140, 142. As shown in FIG. 2A, voltage source 200 includescharge pumps 140 and 142 for generating the signal supply voltages +Vsand −Vs, respectively, and a charge pump regulator 210. Note that chargepump regulator 210 is not shown in FIGS. 1A, 1B, and 1C for simplicityof illustration, but is present with voltage regulators 140, 142. Chargepump regulator 210 is coupled to the signal supply voltages +Vs and −Vs,and receives a reference voltage Vref that represents a desired voltagelevel for Vs. Charge pump regulator controls or regulates the chargepumps 140 and 142 using control signals 42 b and 42 a, respectively.Charge pumps 140 and 142 are each coupled between first and second powersupply voltages (e.g., Vdd and GND), and are configured to draw currentfrom the power supply to provide the signal supply voltages +Vs and −Vs,and to maintain the signal supply voltages +Vs and −Vs at desired levelsin response to control signal φ1 and to control signals φ2b and φ2a,respectively.

FIG. 2B illustrates an embodiment of charge pump regulator 210. As shownin FIG. 2B, charge pump regulator 210 receives a pair of non-overlappingclocks φ1 and φ2 (or, it may include a clock signal source 220 thatgenerates the pair of non-overlapping clocks φ1 and φ2). A pair ofequally sized resistors 216 and 218 divides a voltage difference between+Vs and −Vs to produce a voltage Vcm (a common-mode voltage between thepositive and negative signal supply voltages). The signal supplyvoltages +Vs and −Vs are nominally symmetric about GND. A digital or“bang-bang” control loop including sense amplifier/latch 224 amplifiesthe difference (error) voltage between Vcm and GND. When the voltage onVcm is higher than the voltage on GND, a signal on line 228 is asserted,and during the following φ2 interval, AND gate 234 outputs signal φ2athat tracks signal φ2 (i.e., signal φ2a is asserted if signal φ2 isasserted). On the other hand, if the voltage on Vcm is lower than thevoltage on GND, sense amplifier/latch 224 amplifies this difference andde-asserts the signal on line 228, such that φ2a is not asserted.

Likewise, digital or “bang-bang” control loop including senseamplifier/latch 222 that amplifies the difference (error) voltagebetween +Vs and Vref. If the voltage on +Vs is lower than the voltageVref, a signal on line 226 is asserted, and during the following φ2interval, AND gate 232 outputs signal φ2b that tracks signal φ2 (i.e.,signal φ2b is asserted if signal φ2 is asserted). On the other hand, ifthe voltage +Vs is higher than the voltage Vref, sense amplifier/latch222 amplifies this difference and de-asserts the signal on line 226,such that φ2b is not asserted. By asserting and deasserting φ2a and φ2bbased on comparison of Vcm with GND and +Vs with Vref, respectively, thesignal supply voltages +Vs and −Vs can be maintained at the desiredlevels, as discussed below.

FIG. 2C illustrates another embodiment of charge pump regulator 210.Here, the signal supply voltage +Vs is fed back into comparator 242 forcomparison with the reference voltage Vref. The output 246 of comparator242 is asserted if +Vs is smaller than Vref. Thus, the output φ2b of ANDgate 252 follows the clock signal φ2 when +Vs is lower than Vref and isnot asserted otherwise. Likewise, the signal supply voltage −Vs is fedback into window comparator 244 for comparison with the referencevoltage Vref. The output 248 of window comparator 244 is asserted if thedifference between GND and −Vs is smaller than the difference betweenVref and ground (i.e., GND-(−Vs)<Vref-GND, or Vs is smaller than Vref).Thus, the output φ2a of AND gate 254 follows the clock signal φ2 whenthe difference between GND and −Vs is smaller than the differencebetween Vref and GND. Otherwise, it is not asserted.

FIGS. 2D and 2E illustrate embodiments of charge pumps 140 and 142,respectively. Referring to FIG. 2D, charge pump 140 includes a switchingdevice 262 (shown as a P-type metal-oxide-semiconductor (MOS) fieldeffect transistor (PFET)) that is turned on or off according to clocksignal φ1, and a switching device 264 (shown as an N-type MOS fieldeffect transistor (NFET)) that is turned on or off according to clocksignal φ2b. Charge pump 140 further includes a capacitor 266 and acapacitor 268. Capacitor 266 has a much smaller capacitance than that ofcapacitor 268 and is therefore sometimes referred to as Csmall, whilecapacitor 268 is sometimes referred to as Cbig. Cbig 268 is coupledbetween +Vs and GND, so the voltage across capacitor 268 is the signalsupply voltage +Vs.

FIG. 2F is timing chart showing signals φ1, φ2, φ2b, the voltage acrosscapacitor 266 (V_Csmall), and the voltage across capacitor 268 (+Vs) incomparison with Vref during a series of time periods after IC deviceincluding the charge pumps is turned on. As shown in FIG. 2F, duringtime period t1, when φ1 is asserted, switch 262 is turned on, allowingcharge to flow from Vdd to capacitor 266, so that a voltage acrosscapacitor 266, V_Csmall, increases to a level between Vdd and GND. Thevoltage across Cbig is lower than Vref so φ2b is tracking φ2. Duringtime period t2, when φ1 is de-asserted while φ2 and thus φ2b isasserted, switch 262 is turned off while switch 264 is turned on, sothat stored charge in Csmall is shared with Cbig and the voltage acrosscapacitor 268, +Vs, increases while V_Csmall decreases.

Since the capacitance of Csmall is much smaller than the capacitance ofCbig, and the increase in voltage across Cbig during time period t2 ismuch smaller than the decrease in voltage across Csmall, so the voltageacross Cbig at the end of time period t2 is slightly higher than thevoltage across Cbig at the start of time period t2. The above chargingof Csmall and subsequent charging of Cbig may repeat in response tosignals φ1 and φ2b, until the signal supply voltage +Vs reaches or ishigher than Vref at the end of time period t3. As discussed above, aslong as +Vs remains at or above Vref during time periods t4 and t4, φ2bis not asserted, meaning switch 264 does not turn on and Cbig does notget further charged by Csmall. When +Vs drops below Vref during timeperiod t6, φ2b follows φ2 again in time period t7, and charges fromCsmall will be shared with Cbig until +Vs is higher than Vref again.

The signal and voltage curves shown in FIG. 2F are for illustrativepurposes only and are not to replicate or scale with signals or voltagesin a real IC device. In some embodiments, the voltage between Vdd andGND is much higher than Vref. For example, Vdd may be about 1 volt,while Vref is about 0.1 volt. In such case, if the charge pumps run at afrequency of, for example, 1 GHz, data is being driven out at, forexample, 4 Gbps, and the impedance of transmitter 102 (thus theimpedance of the load) is, for example, 50 ohms, Csmall can be about0.75 pF to about 1.25 pF while Cbig can be about 50 pF to about 250 pF.

On the other hand, charge pump 142 includes switches 272, 282 that areturned on or off according to clock signal φ1, and switches 274, 284that are turned on or off according to clock signal φ2a. Charge pump 214further includes a capacitor Csmall 276 and a capacitor Cbig 278. Cbig278 is coupled between GND and −Vs so the voltage across Cbig 278 is thesignal supply voltage −Vs. Switches 272, 282 are turned on when clocksignal φ1 is asserted so that charge is extracted from power supply Vddvia switch 272 and 282 and stored in capacitor Csmall 212. Switches 274and 284 are turned on when signal φ2a is asserted and signal φ1 isde-asserted, so charges stored in Csmall is pumped into Cbig. Thisprocess repeats until the common mode voltage Vcm is lower than GND, orthe difference between GND and Vs is larger than the difference betweenVref and GND. As discussed above in association with FIGS. 2B and 2C, aslong as the common mode voltage Vcm remains lower than GND, or thedifference between GND and Vs remains larger than the difference betweenVref and GND, φ2a remains de-asserted and no charge is pumped fromCsmall 276 to Cbig 278 to further increase the voltage across Cbig 278.When the common mode voltage Vcm becomes higher than GND, or thedifference between GND and Vs becomes smaller than Vref, φ2a follows φ2again and Cbig 278 gets charged from Csmall until Vcm becomes lower thanGND, or the difference between GND and Vs becomes larger than thedifference between Vref and GND again.

FIGS. 2D and 2E show that switches 262 and 272 are implemented usingP-type metal-oxide-semiconductor field effect transistors (P-MOSFET orPFET), while switches 264, 274, 282 and 284 are implemented using N-typeMOSFETs (NFET). PFET 262 or 272 is turned on when φ1 is asserted byvirtue of inverter 260 or 270, respectively, which inverts the sense ofφ1 to drive the gate terminal of PFET 262 or 272. Those skilled in theart will easily understand that other types of switches can be used inplace of the MOSFETs. The capacitors 266, 268, 276, and 278 may beimplemented using MOS capacitors, but other types of capacitors canalternatively be employed.

The embodiments of FIGS. 2A through 2F are shown by way of examples.There are many other possible embodiments of on-chip voltage sources orregulators. For example, some embodiments may have additional Csmallcapacitors and additional switches that charge these Csmall capacitorsin a series circuit arrangement and discharge the Csmall capacitors in aparallel arrangement, thereby providing higher overall efficiency thanthat of the simple charge pumps of FIGS. 2D and 2E. In a furtherexample, instead of using clocks to gate the MOSFETs on and off in thecharge pumps, other type of switches that respond to othercharacteristics of control signals such as frequency, amplitude or dutycycle of the control signals may be used. In yet another example,switched capacitors may be used in place of the switch and capacitorcombinations in the charge pumps. In still other examples, otherregulating means such as a linear regulator or switching regulator maybe used to control the signal supply voltages.

In one embodiment, charge pumps 140 and 142 are implemented withcomponents having substantially the same efficiency. In the embodimentshown in FIG. 2A, charge pump 140 draws current from Vdd whentransmitter 102 is transmitting “1”s while charge pump 142 draws currentfrom Vdd when transmitter 102 is transmitting “0”s. Thus, assuming thatthe transmitter 102 has matching impedance with the transmission lines132, 130, the transmitter 102 draws approximately the same amount ofsupply current from the power supply regardless of which way thetransmitter 102 is driving the current (i.e., sourcing current fromtransmitter 102 to receiver 104, or sinking current from receiver 104 totransmitter 102). Thus, to the extent that charge pumps 140 and 142 haveabout the same efficiency, the complementary configuration of thevoltage source(s) 140 ensures that supply current variation in signalingsystem 100 is near zero, thereby reducing or eliminating simultaneousswitching noise (SSO) inherent in conventional single-ended signalingschemes.

The embodiments of FIG. 2A through 2F are shown by way of example only.There are many other possible embodiments of on-chip voltage regulators.

FIG. 3 illustrates an example of a common-gate amplifier that can beused as the receiver amplifier(s) in the data transmission system ofFIGS. 1A through 1C when GND is chosen as the reference voltage,according to one embodiment. Current source 906 establishes a referencecurrent in NFET 914, thereby generating gate reference voltage Vcas.NFETs 910 and 912, which are the amplifying transconductances inamplifier 306, are drawn at the same shape factor (width/length) as 914so that they carry about the same current as 914, when their inputs,driven into their source terminals, attached to pins 136 and 138, are atsubstantially the same potential, since in this case the gate-to-sourcevoltages (Vgs) for all three transistors are substantially equal and theshape factors are identical. The data signal, attached to pin 136, iscompared in amplifier 306 with the signal reference voltage, input onpin 138 from the power supply GND plane 130. The current from line toGND develops a voltage across termination resistor 116; this voltageswings from about +Vs/2 to about −Vs/2. When the voltage on line (pin136) is higher than the voltage on GND (pin 138) NFET 910's Vgs is lessthan Vgs at NFET 912. Therefore, less current flows through 910 andthrough 912, and the voltage drop through load resistor 902 is smallerthan the voltage drop through load resistor 904. This causes the voltageon outH to rise above the voltage on outL. On the other hand, if thevoltage on 136 is lower than the voltage on 138, then NFET 910's Vgs isgreater than Vgs for NFET 912. Consequently NFET 910 carries morecurrent than 912, and the voltage across 902 is larger than the voltageacross 904. In this case, the voltage on outH is less than the voltageon outL. Under these circumstances the current driven back into line 132(pin 136) by NFET 910 is larger than the current driven into the line132 by 910 when the voltage on line exceeds the voltage on GND. Toprevent this current unbalance from introducing an unwanted voltageoffset in amplifier 306, a compensating resistor 922, RC, is used toreturn some of the current to the negative power supply −Vs.

Still referring to FIG. 3, the magnitude of the voltage between outH andoutL is higher than the magnitude of the voltage between line (pin 136)and GND (pin 138) thanks to the gain of amplifier 306; the gain ofamplifier 306 is about Gm×RL, where Gm is the transconductance of NFETs910 and 912, and RL is the resistance of load resistances 902 and 904.

Still referring to FIG. 3, the terminating impedance presented acrosspins 136 and 138 is the parallel combination of the terminating resistorRT 116, the compensating resistor RC 922, and the input impedance of theamplifier, which is about 1/Gm, where Gm is the transconductance oftransistors 910 and 912. This parallel combination of resistances shouldbe adjusted to about the characteristic impedance of the transmissionline formed by line 132 and GND to avoid reflections in the transmissionline.

The foregoing embodiment of the input amplifier is shown by way ofexample. Many alternative embodiments of input amplifiers can beemployed in receiver 104 or 144 in system 100.

Upon reading this disclosure, those of skill in the art will appreciatestill additional alternative designs for forwarding signal supplyvoltages in a data transmission system. Thus, while particularembodiments and applications have been illustrated and described, it isto be understood that the disclosure herein is not limited to theprecise construction and components disclosed herein and that variousmodifications, changes and variations may be made in the arrangement,operation and details of the method and apparatus disclosed hereinwithout departing from the spirit and scope of the disclosure herein.

What is claimed is:
 1. A circuit, comprising: one or more voltageregulators configured to generate at least one signal supply voltage; atleast one terminal configured to couple the at least one signal supplyvoltage to a voltage forwarding mechanism, for forwarding the at leastone signal supply voltage to another external circuit; and a transmitterconfigured to transmit a first signal having a signal swing dependent onthe at least one signal supply voltage to said another external circuitvia a communication link.
 2. The circuit of claim 1, wherein the firstsignal is referenced to a power supply voltage and swings substantiallysymmetrically with respect to the power supply voltage.
 3. The circuitof claim 2, wherein the power supply voltage is a ground voltage.
 4. Thecircuit of claim 1, further comprising: a receiver configured to receivea second signal transmitted by said another external circuit; and afeedback analyzer circuit configured to detect a parameter indicative ofsignal integrity of the second signal, said one or more voltageregulators adjusting the at least one signal supply voltage according tothe parameter.
 5. The circuit of claim 4, wherein said one or morevoltage regulators are configured to adjust the at least one signalsupply voltage so as to increase the signal swing of the first signalresponsive to increased noise in the second signal.
 6. The circuit ofclaim 4, wherein the feedback analyzer circuit is a bit error ratedetection circuit and the parameter is a bit error rate of the secondsignal.
 7. The circuit of claim 1, wherein the circuit is a memorycontroller and said another external circuit is a memory device.
 8. Acircuit, comprising: at least one terminal configured to receive atleast one signal supply voltage forwarded from another external circuitvia a voltage forwarding mechanism; and a transmitter configured totransmit a signal having a signal swing dependent on the at least onesignal supply voltage to said another external circuit via acommunication link.
 9. The circuit of claim 8, wherein the signal isreferenced to a power supply voltage and swings substantiallysymmetrically with respect to the power supply voltage.
 10. The circuitof claim 9, wherein the power supply voltage is a ground voltage. 11.The circuit of claim 8, wherein the circuit is a memory device and saidanother external circuit is a memory controller.